This invention relates to a method of treating the surfaces of shaped articles of polycarbonate resins to make the surfaces free of scratches.
Transparent plastic articles, for example, shaped articles of polycarbonate, poly(methyl methacrylate), cellulose butyrate, polyvinyl chloride and polystyrene, have heretofore been considered useful in many applications as glass substitutes because of their transparency and light weight. They have had the defect, however, that they can be put to only limited applications because they are susceptible to scratch and tend to lose transparency. To remove this defect, a method has been suggested which involves coating a transparent paint on the surface of the transparent plastic article to form a protective film.
In U.S. patent application Ser. No. 869,863 filed on Jan. 16, 1978 now U.S. Pat. No. 4,167,537, the present inventors provided a paint which comprises an amino-containing silane derivative, an epoxy-containing silane derivative, a melamine-alkyd resin, and an acrylic acid derivative, which can be applied to various transparent plastic articles, and which can satisfy both scratch resistance and durability. This paint was superior in adhesion, scratch resistance, and durability in dipping tests using warm water and was by far superior to conventional paints in weatherability as well, but posed the problem in storage stability that its viscosity increased during a relatively short period of time and finally the paint as a whole gelled.
In an attempt to solve the problem in storage stability of that paint, the present inventors traced the causes, and found that melamine being a component of the paint easily reacts with another component, a reaction mixture of the reaction of an amino-containing silane derivative with an epoxy-containing silane derivative, thereby causing the increase in the viscosity of the paint and its gelation. To improve the storage stability of the paint to a level which would cause no trouble to its actual use (one month or longer, if the storage stability is expressed by a period), the melamine content of the melamine-alkyd resin must be set at less than 10 wt%. The melamine content of the melamine-alkyd resin set at less than 10 wt%, however, has resulted in a marked reduction in the hot water resistance of the resulting coated film.
The present inventors made eager studies on improving the storage stability of the paint disclosed in U.S. patent application Ser. No. 869,863 now U.S. Pat. No. 4,167,537, and on a method capable of maintaining on a high level the durability of an article coated with the improved paint. These studies lead them to the present invention.